1. Field of the Invention
The present invention relates to a gallium nitride (GaN) semiconductor light emitting device (LED), and more particularly to a high quality GaN semiconductor light emitting device and a method of manufacturing the same, which comprises a buffer layer consisting of single crystal AlN and which causes a reduction of defects, such as Ga vacancies and dislocations caused by lattice mismatching, by Al doping when growing GaN, thereby enhancing electrical and optical properties.
2. Description of the Related Art
Recently, a light emitting device display board developed as a new transmission media for images or information has been advanced to a level of displaying a moving image, such as various CF images, graphic images, video display, etc., starting from information of simple characters or figures in the early days of the LED display board. With regard to colors, as a high brightness blue LED using a nitride semiconductor has emerged recently, it has become possible to exhibit a full color display using colors of red, yellow-green and blue, not limited to an existing monochromatic coarse display or at most a limited range of colors, such as red or yellow-green, in the past. Nevertheless, the yellow-green LED has a lower brightness than the blue LED or the red LED and emits light having a wavelength of about 565 nm which is not the wavelength of green required for the three primary colors. Thus, it is not possible to display the full range of natural colors. These problems are overcome by a high brightness pure green nitride semiconductor LED emitting a wavelength of 565 nm suitable for displaying the full range of natural colors.
Such nitride semiconductors use a nitride semiconductor material with the formula AlxInyGa(1−x−y)N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1), and investigations are being actively undertaken particularly on the semiconductor LED using GaN. Meanwhile, although a sapphire substrate is generally used as a dielectric substrate in the nitride semiconductor LED, there is no commercially available substrate which has an identical crystal structure to that of a nitride semiconductor material, such as GaN, and which is in lattice matching with the material. Thus, crystal defects are created due to the differences in lattice parameters and in thermal expansion coefficient between the sapphire substrate and a GaN layer grown on the sapphire substrate. In order to prevent these defects, conventionally, after a GaN buffer layer grown at a low temperature is formed on the sapphire substrate, a GaN layer is grown on the buffer layer at a high temperature. This process is provided to decrease the difference in lattice parameters between the sapphire substrate and the GaN layer.
Nevertheless, the buffer layer grown at a low temperature has a great number of crystal defects and properties closer to a non-crystalline structure rather than a crystalline structure. Thus, if the GaN layer is directly grown at a high temperature on the buffer layer grown at the low temperature, a large number of crystal defects move toward the upper GaN layer grown at the high temperature, thereby creating the defects, which are referred to as “dislocations.”
Conventionally, in order to grow the GaN layer free of dislocations, the LEO (Lateral Epitaxy Overgrowth) method [also referred to as ELOG (Epitaxial Lateral Overgrowth) method] or the pendeoepitaxy method has been suggested. Both methods prevent defects created at the interface between the sapphire substrate and the GaN layer from moving upward by laterally growing the GaN layer. Specifically, in the LEO method, after a dielectric mask is formed on the sapphire substrate, or on the GaN epitaxial layer primarily grown on the sapphire substrate, the GaN layer is overgrown on the portion where the mask is not formed, such that the overgrowing GaN layer can grow laterally. Meanwhile, according to the pendeoepitaxy method, after the primary growth of the GaN epitaxial layer on the sapphire substrate and the formation of the masks on the primarily grown GaN epitaxial layer, as is similar to the LEO method, the mask is partially etched and formed with grooves, so that the GaN epitaxial layer is overgrown on the grooves.
FIGS. 1a to 1d show a method of growing the GaN layer with the conventional LEO method as described above. According to the LEO method, as shown in FIG. 1a, a GaN epitaxial layer 11 is primarily grown on a sapphire substrate 10, and as shown in FIG. 1b, a mask 12 with predetermined patterns is formed on the GaN epitaxial layer 11 using a silicon oxide film or a silicon nitride film. As shown in FIG. 1c, the GaN is overgrown on the portion where the mask 12 is not formed. On the mask 12, a GaN layer 13 is laterally grown, as indicated by an arrow of FIG. 1c. As the lateral growth of GaN is completed, the growth of the GaN layer 13 is completed, as shown in FIG. 1d. In addition to the above steps of the LEO method, the pendeoepitaxy method further comprises the step of etching to remove the portion of the GaN epitaxial layer which is not covered with the mask, after forming the mask.
It is generally known that the dislocations moving in the GaN layer formed by the LEO method or by the pendeoepitaxy method tend to be reduced. As shown in FIG. 2, in the portion to which the primarily grown epitaxial layer 11 is exposed, the underlying dislocations move to the overgrowing GaN layer 13, while in the portion covered with the mask 12, the GaN layer 13 is laterally overgrown and no underlying dislocations move, thereby reducing the defects.
However, in case of growing GaN with these methods, there are problems that the dislocations A in the portion which the mask does not cover continue to move upward, and that a high density of dislocations B is created at the interface where the laterally overgrowing GaN layers 13 meet. Further, the defects are created due to the stress generated between the material of the mask 12 and the overgrown GaN layer 13. Due to the defects caused by such dislocations, the electrical and optical properties of the nitride semiconductor device are deteriorated and the yield is reduced.
In addition to these problems, the step of preparing the mask in the above methods caused increased costs, and the addition of the steps of patterning and overgrowing after the primary growth of the GaN epitaxial layer complicate the process.
As described above, even though the LEO method or the pendeoepitaxy method is applied so as to reduce defects caused by lattice mismatching, there are problems that the defects, such as dislocations, cannot be significantly reduced and the added steps result in a complex process and increased costs. Thus, there is a need in the art to provide a new GaN semiconductor LED and a method of manufacturing the same, which exhibits excellent electrical and optical properties by preventing the defects, such as dislocations caused by the lattice mismatching between the sapphire substrate and the nitride semiconductor material, such as GaN.